Draft apparatus



. 10 116, 1957 A. H. IIQAWDON, JR 1 2,799,482

DRAFT APPARATUS Filed Dec. 28. 1954 INVENTOR. F AL BERT H. Rn woo/ m.

Uflitfid States Patent DRAFT APPARATUS Albert H. Rawdon, Jr., Shrewsbury, iMass., assignor to Riley Stoker Corporation, Worcester, Mass, a corporation of Massachusetts Application December 28, 1954, :Serial No. 478,101

4-Claims. Cl. 257-444 This invention relates generally to :a draft apparatus, and more particularly, to an arrangement of a fan and associated equipment for use with the :air heater of .a steam .generating apparatus.

It is common practice in the operation of asteam generating apparatus to increase the efliciency of the unit by passing exhaust gases in heat relationship to the incoming combustion air. However, the air heater in which this heat exchange takes place is often subject to corrosion difliculties, particularly because of the fact that the temperatures in the exhaust .gases are at such :a low value that condensation of corrosive substances takes place. In the past this difiiculty has :been obviated by causing a portion of the air leaving the air heater to be recirculated to the fan supplying air to the air heater so that the :air entering the air heater from the fan is at a high enough temperature to reduce corrosion difiiculties. Returning this air for recirculation, however, is not without difiiculties. In the first place, the operation of a fan .is considerably handicapped by the introduction of any obstruction into its entrance opening. Therefore, in the past, many methods have been proposed for introducing recirculation air to the fan Without unduly obstructing the entrance opening thereof. For instance, in the pateat to Cassie, No. 2,488,267, and the patent to Geissler et al., No. 2,476,543, means is provided for removing the recirculation duct from in front of theopening to the 'fan when recirculation is not taking place. In the constructions of the prior art, it has been difiicult to obtain intermediate amounts of recirculation without loss of air into the atmosphere. Many of the difiiculties experienced with the prior art have been removed by the present invention.

It is therefore an outstanding object of the present invention to provide a draft apparatus for use with the air heater of a steam generating apparatus which will permit air recirculation at intermediate amounts of recirculation without substantial loss of air into the atmosphere.

Another object of the present invention is the provision of a draft apparatus for use with an air recirculation system for an air heater in which the hot recirculated air does not impinge on the the shaft bearings of the fan.

It is another object of the present invention to provide a draft apparatus for use with a recirculation system wherein there is little danger of spillage of the hot recirculated air into the atmosphere around the apparatus or of danger to operating personnel.

It is a still further object to provide a draft apparatus for use in a recirculation system in which an unusually high degree of suction is produced in the recirculation duct to promote recirculation even at low load operation of the steam generating unit.

Although the novel features which are believed to be characteristic of this invention will be particularly pointed out in the claims appended hereto, the invention itself as to its objects and advantages, the mode of its opera- 'tion and the manner of its organization may be better understood by referring to the following description taken in connection with the accompanying drawings forming a part thereof in which:

Figure 1 is an elevational view of the apparatus looking along the axis-of the fan,

Figure 2 is an elevational view of the apparatus taken transversely to the axis of the fan with portions broken away, and

Figure 3 is a perspective View of an element of the apparatus of the invention.

Referring to Figures 1 and 2, a fan 10 is shown in use in conjunction with an air heater 11. The air heater 11 is shown more or less schematically, although it is to be realized that in actual practice the air heater will be considerably larger than is indicated in the drawings. The air heater 11 consists primarily of a series of tubes 12 connected between a tube sheet 13 and a tube sheet 14. A gas duct '15 coming from a steam generating unit, not shown, is connected to one side of the air heater so thatthe gases passing therethrough pass internally of the tubes 12. A duct 16 is connected to the other side of the air heater to receive the gases after they have passed through the tubes 12 and is connected at its outer end to a stack, not shown. The fan 10 consists of a scroll-like housing 17 having a circular opening 18 therein, a fan shaft 19 through which the blades of the fan, not shown, are attached passes through the housing concentrically with the opening 18 and is mounted at its ends in journal bearings 20 and 21 mounted on pedestals 22 and 23 respectively. The fan discharges into a duct 24 which duct is connected at its other end to the air heater 11. The duct is connected to the air heater in such a Way that the air will pass transversely of the tubes 12 externally thereof in heat exchange relationship with the gases passing internally of the tubes. Another duct 25 is connected to the other side of the air heater 11 to receive the air which has passesd therethrough. The duct 25 leads to the burners and other apparatus of the steam generating unit. Dampers 26 are situated in the duct 24 and are mounted on shafts 27 by which they may be manipulated. These dampers are capable of closing 01f the entire area of the duct, if necessary. A duct 28 is connected to the duct 25 just beyond the air heater 11 and is connected at its other end to the housing 17 of the fan at a position adjacent the opening 18. As a matter of fact, the lower edge of the duct 28 is situated exactly in line with the edge of the opening 18. Dampers 29 mounted on actuating shafts 30 are capable of cutting off all flow through the duct 28. A shaft 31 is mounted in bearings 32 and 33 located adjacent to the face of the housing 17 and is tangential to the circular opening 18 in the portion thereof occupied by the duct 28. On this shaft is mounted a director element 34.

Referring to Figure 3 it can be seen that thedirector element 34 consists of a tubular member 35 lying at the apex of circular-sector end plates 36. The arcuate portions of the end members 36 are joined by a plate 37 formed to give a cylindrical surface. One edge of each of the end members 36 and the plate 37 lie in a common plane and are provided with a flange 38.

Referring again to Figure 2 it can be seen that the angles subtended by the sides of each of the sector end members 36 is approximately degrees and that the radius of each of these segments is substantially equal to the dimension of the rectangular duct 28 measured in the direction of the axis of the fan. When the director element 34 is in the position shown in Figure 2, it can be seen then that the portion of the flange which is connected to the plate 37 is closely adjacent to the side of the duct 28 which is furthest away from the fan. The

lower edge of this side is provided with a seal 39 on which the flange 38 rests and which serves to limit the motion of the director element 34 in a downward direction. The overall length of the director element 34 and the length of the tubular portion 35 is approximately the same as the width of the duct 28'andthessides of the duct 28 are provided with sealswhich are to be contacted by the flange 38 in this portion also. It should be noted that the seal 39 is such that when the director element 34 is rotated into other positions by means, of the shaft 31 to which the tubular element 35 is keyed, the seal 39 brushes lightly against the outer surface of the plates 36 and 37 thereby sealing the junction of the air duct and the director element against leakage. p

The operation of the apparatus will be clearly understood in view of the above description. Gas from the steam generating unit enters the air heater 11 from the duct 15 and passes into the duct 16 for disposal. The fan shaft 19 is rotated by means not shown and a blast of air passes through the duct 24, the air heater 11, and the duct 25 on its way to the steam generating unit. Depending on the settings of the dampers 29, a small portion of the air enters the duct 28 and passes downwardly toward the fan again. When it is desired to recirculate the maximum amount of heated air, the director element 34 is in the lowermost position shown in the drawings and the air is picked up by the fan mixed with the colder gases entering through the opening 18 and is passed upwardly into the duct 24. If it is not necessary to recirculate any of the air at a certain operation of the steam generating unit, the director element is raised into the dotted position where it acts as a damper to any flow of gas down the duct 28. In this raised position, it can be seen that the opening 18 is entirely free of obstruction. The director member may be used in intermediate positions when it is not desired to recircul-ate the maximum amount of air or to prevent any recirculation whatsoever.

While certain novel features of the invention have been shown and described and pointed out in the annexed claims, it will be understood that various omissions, substitutions, and changes in the forms and details of the device illustrated and in its operation may be made by those skilled in the art without departing from this spirit of the invention.

It is evident that the size of the lower portion of the duct 28 and the director element 34 may be made such that in the lower position the director element will cover a substantial portion of the fan opening 18. This will produce a high suction in the duct 28 and enhance the recirculation of hot air; this is particularly important at low load operation of the steam generating unit, during which operation such recirculation is normally difiicult to obtain.

The invention having been thus described, what is claimed as new and desired to secure by Letters Patent is:

1. A draft apparatus for use with an air heater and a. fan housing having an inlet opening therein, comprising a duct of rectangular cross-section adapted to receive a portion of the air leaving the air heater and to return it to the inlet opening, the duct having an end adjacent the inlet opening, a director element pivotally mounted with its pivotal axis adjacent the edge of the said opening, the director element consisting of two similar circular-sector ends having their apices at the pivotal axis and a cylindrical plate joining the circular edges of the ends, the director element being movable from a position wherein it resides entirely within the lower end of the duct and prevents flow of recirculated air therefrom to a position wherein it resides outwardly of the duct and causes all air passing .through the duct to enter the fan through the inlet opening.

2. An apparatus as recited in claim 1, wherein the edge of the director element which is innermost of the duct is provided with a flange and the edge of the said end of the duct is provided with a seal to be engaged by the said flange when the director element is in the outward position.

3. An apparatus as recited in claim 2, wherein the arcs subtended by the circular sector ends are degrees and the radius thereof is sufficient to cause a portion of the said seal to slide along the cylindrical plate as the director element moves from one position to another. 1

4. An apparatus as recited in claim 1, wherein the ends of the director element are so spaced that portions of the said seal slides along the surfaces of the ends as the element moves from one position to another.

References Cited in the file of this patent UNITED STATES PATENTS 728,281 Noel May 19, 1903 2,476,543 Geissler et a1. July 19, 1949 2,665,840 Powell Jan. 12, 1954 

